if you have an iOS device with AirPlay around (basically, anything starting with the iPad 1 / iPhone 4 / iPod touch 4), you can use it to stream AppleTV-native files (mov / mp4 / m4v) from any UPnP source to an Apple TV. All this without having to power up a much more power-hungry x64 / x86 desktop (notebook) computer. (Just compare the power usage of an iPod touch to that of, say, a traditional X86 desktop...)

Basically, you'll need to fire up one of the following UPnP input- and AirPlay output-capable players, connect to the ATV and start playback from the UPnP source:
nPlayer, PowerPlayer, 8player, AcePlayer, BUZZ Player, GoodPlayer, CineXPlayer, GPlayer by Ginkgo Tech

That's all you'll need to stream. Via a proxy, yes, but it, after all, doesn't require a full x86 computer.